Materials processing in a low gravity environment has been made possible by the advent of a space shuttle orbiter. Prior to this event it was not possible to grow crystals or deposit thin films by vapor phase methods in a low gravity environment (1 meter/sec.sub.2 or less residual gravitational acceleration). There does not appear to be any prior art dealing with closed chamber physical vapor transport growth of organic films even under unit gravity conditions.
Background art involving low gravity materials processing by vapor phase methods are the physical vapor transport experiments flown on the Space Shuttle Orbiter, and earlier Skylab and Appolo-Soyuz missions, in which bulk crystals of alpha-HgI.sub.2 and GeSe were grown by physical vapor transport [see L. van Den Berg and W. F. Schnepple, in Materials Processing in the Reduced Gravity Environment of Space , ed. Guy E. Rindone, Elsevier, N. Y., (1982) 439; H. Wiedemeier, F. C. Klaessig, E. A. Irene, and S. J. Wey, J. Cryst. Growth 31, 36 (1975); H. Wiedemeier, H. Sadeek, F. C. Klaessig, M. Norek and R. Santandrea, J. Electrochem. Soc. 124(7), 1095 (1977); F. Langlais, J. C. Launay, M. Pouchard and P. J. Hagenmuller, J. Crystal Growth 62 (1986) 145; H. Wiedemeier, D. Chandra, and F. C. Klaessig, J Crystal Growth 51 (1980) 345; and E. Kaldis, R. Cadoret and E. Schonherr, Chapter 11 in Fluid Sciences and Materials Science in Space ed. H. U. Walter, Springer-Verlag (1987)].
Low gravity effects on the mass transport rate of the crystal growth system of Ge-Si-Br to form Ge-Si crystals were studied on "Salyut-6"- "Soyuz" missions [E. Kaldis, R. Cadoret and E. Schonherr, Chapter 11 in Fluid Sciences and Materials Science in Space, ed. H. U. Walter, Springer-Verlag (1987)]. In some cases, in these earth-grown versus space-grown crystal growth experiments, the space grown crystals appeared different or had improved crystal habits, structural morphology or electronic properties.
There appears to be no art wherein closed chamber vapor transport deposition of organic materials is discussed; nor is there art known which describes carrying out vapor transport growth of thin films of any kind in a low gravity environment; nor is art known which describes a vapor transport method, open or closed, to obtain uniaxially oriented organic films.
Existence of eight distinguishable polymorphs of copper phthalocyanine (CuPc) is known in the art. (Two polymorphic forms have very similar lattice structures but slightly differing packing patterns of the molecules in the unit cell.) Most of the crystalline forms have been created as pigment particles, produced by wet chemical or "acid pasting" preparations. The first six pigment forms of CuPc, i.e., alpha, beta, gamma, R, delta-1 and epsilon, were described in U.S. Pat. Nos. 2,486,351, 2,770,629, 3,051,721, 3,150,150, and 3,160,635, and in all cases the X-ray diffraction powder spectra are used to differentiate them. IR spectra are also shown to be sensitive to the alpha, beta, gamma, and R forms (see U.S. Pat. No. 3,051,721) and directly used to define the five alpha, beta, gamma, delta-1, and epsilon types [see B. I. Knudsen, Acta Chem. Scand. 20, 1344 (1966)]. The gamma-form was later proposed by J. M. Assour, J. Phys. Chem. 69(7), 2295 (1965) to simply be highly crystallized large particles of the alpha-CuPc, but Knudsen (supra) in turn provided evidence that the preparation described in U.S. Pat. No. 2,770,629 would produce a distinct polymorph.
The seventh form x-CuPc, was described by J. Sharp and M. Abkowitz, J. Phys. Chem. 77(4), 477 (1973) as the result of rapid sublimation of CuPc into a gaseous ambient at pressures of 1-30 Torr. Again the IR and X-ray spectra were used as evidence.
A recent Japanese Patent disclosure (T. Kawaguchi et al , Patent Kokai No. JP62-77455, Apr. 9, 1987) discusses a vapor deposition method of forming metal-phthalocyanine films to enhance their near infrared absorption characteristics. Their method consists of carrying out conventional sublimation, but in an inert gas atmosphere. Their procedure leads to maximum spectral absorption in the range 774 to 798 nm for 11 metal-phthalocyanines. Details in the translation were lacking, but this appears to be their version of the open chamber flowing gas sublimation technology developed at Xerox Corp. to produce the x-polymorph pigment particles of phthalocyanines by subliming them in an inert gas pressure of 1-30 Torr. An even clearer discussion of this same effect has been published by the Chinese [S. Zurong et al., Kexue Tongbao 31 (16), 1108 (1986)]and predates the Japanese patent work. In the Chinese reference, variation of X-ray diffraction spectra with chamber pressure is discussed as well as absorption spectra. An even earlier reference, Meshkova et al., Opt. Spectrosc. (USSR) 43 (2), 151 (1977)]describes more briefly the same effect. An important fact is that in every one of these prior art cases, deposited CuPc material changed from alpha to x-CuPc as the ambient pressure increased. This is in sharp contrast to the instant invention films grown by closed chamber physical vapor transport (PVT) which show no x-CuPc.
The last distinct polymorph of CuPc disclosed is the delta-2, an orthorhombic form, grown by vapor deposition onto single crystalline graphite and characterized by selected area electron diffraction (SAD) and transmission electron microscopy (TEM) [see H. Saijo et al., J. Crystal Growth 40 (1977) 118-124].
Alpha-CuPc is also called the meta-stable form since it can be thermally converted to the beta-CuPc form [see M. Ashida et al., J. of Crystal Growth -8 (1971) 45-56, designated M. Ishida I]. The latter can be obtained in adequate sized crystals such that a complete crystal structure has been found by C J. Brown, J. Chem. Soc. (A), (1968) 2488, to be monoclinic P2:/a with a=l9.407 A (wherein A=Angstroms), b=4.790A, c=14.628A and beta=120.93 degrees.
Meta-stable alpha-CuPc has not been obtainable in single crystals large enough to do a full crystaI structure determination but it is generally assumed that most of the metal substituted phthalocyanines (Pc's) are isomorphic. This is expressly demonstrated from electron diffraction data for PtPc, CuPc, and ZnPc vacuum sublimation deposited as thin films onto mica [N. Uyeda, M. Ashida and E. Suito, J. of Appl. Physics 36(4) 1453 (1965)]. Isomorphism is also shown using high resolution transmission electron microscopy (TEM) techniques for the metastable forms of Pt, Cu, Co, Fe, Ni and H.sub.2 Pc's, vacuum sublimation deposited onto muscovite [see M. Ashida, et al., Bulletin of the Chemical Society of Japan 39(12) (1966) 2616-2624 designated M. Ishida II]. In particular, the alpha-PtPc form is stable and its X-ray determined crystal structure, monoclinic C2/C, is used as the model for alpha-CuPc [see C. J. Brown, J. Chem. Soc. (A), 1968, 2494]. This space group was consistent with the TEM/SAD data of Ashida et al. II, supra who determined lattice parameters for the alpha-CuPc in thin film form as a=25.92 A, b=3.79 A (wherein A=Angstroms), c=23.92A and beta=90.4 degrees.
The situation may be more complicated than this since even Brown [see C. J. Brown, J Chem. Soc. (A), 1968, 2494 (1968)]describes a second non-beta form of PtPc. Kobayashi et al. Acta Cryst. A37, 692 (1981) propose on the basis of minimum exposure TEM images that ZnPc has at least three meta-stable alpha-polymorphs, and Debe (see M. K. Debe, J. Appl. Phys. 55(9), 3354 (1984) and Erratum, J. Appl. Phys. 62(4), 1546 (1987) has shown that a second meta-stable thin film form of H.sub.2 Pc exists. Finally, Murata et al. (Y. Murata, J. R. Fryer and T. Baird, J. of Microscopy, 108 (3) (1976) 261-275) have also used minimum exposure TEM imaging of individual CuPc molecules stacked in very thin films on KCl to suggest that isomorphism with alpha-PtPc was not being adhered to.
Hence when vacuum sublimation deposited onto substates near room temperature, the metastable alpha-CuPc form is generally obtained and those instances when other metastable forms are obtained may simply reflect the influence of the substrate on the growth of a film when the lattice energy is comparable to the interface energy between the film and substrate molecules. Indeed, the delta-2 CuPc reported by Saijo et al. (supra) is a manifestation of this.
The most commonly obtained crystal form of a vacuum sublimation deposited film of CuPc is the metastable alpha-CuPc for which the crystal structure and lattice constants are as above (Ashida et al., II). The X-ray diffraction spectrum from a randomly oriented sample of crystallites formin9 a film, or a powder sample, would appear as in FIG. 1. Implied interplanar spacings and relative peak intensities are quite consistent among the various references which show such X-ray spectra of alpha-CuPc (see U.S. Pat. Nos. 3,051,721 and 3,160,635, and Assour, supra, and Sharp et al., supra) and the interplanar spacings derived from electron diffraction of the CuPc thin films (see Ashida et al., III supra).